Photo-ionization detection (PID) is a well-established method to detect volatile organic gas. The basic design of PID detectors includes a high-energy photon source, an ionization chamber, and a pair of electrodes. Typically, a gas discharge ultraviolet (UV) lamp is used as the high-energy photon source. This lamp produces photons with photon energy of 9.2 eV and above. When such high-energy photons hit organic gas molecules, molecules having ionization levels below the photon energy are ionized. In most PID instrument designs, the organic gas is brought into an ionization chamber by a pump. A UV lamp illuminates the chamber with high-energy photons. The resulting ions will cause a current flow between two electrodes disposed inside the chamber. An electrometer is used to measure the current. The current measurement can be converted into concentration in parts per million (ppm) of the organic gas based on the flow rate of the gas stream.
In a classical PID design, a glow discharge UV lamp is used to produce high-energy photons. The lamp is constructed with two electrodes placed inside a sealed glass envelope. The glass envelope is filled with certain gases, such as helium, argon or krypton. High voltage is applied between the two electrodes to induce an ionization process (i.e., separation of the electrons from the molecules). The ions and electrons are recombined shortly after to generate photons. These photons then pass through a UV window in the glass envelope to illuminate an associated ionization chamber.
A classical ionization chamber is constructed with an airtight housing and a pair of closely-spaced electrodes. The gas is introduced to the chamber through a small gas inlet and leaves the chamber through a gas outlet. The two electrodes are generally arranged in concentric form with one electrode in the middle of a cylindrical shaped electrode. A high-voltage DC (&gt;150 V) is applied between the two electrodes to generate a high electric field. The UV window of the lamp is placed directly over the space between the two electrodes. When the gas molecules enter the chamber, they are ionized by the photons from the UV lamp. The resulting ions and electrons will be attracted to the two electrodes by the electric field. An electrometer measures the current flow.
The ionization chamber of the prior art has several disadvantages. The distance from the center electrode to the outside cylindrical electrode is relatively large (about the radius of the UV window). In order to achieve high electric field strength, relatively high voltage needs to be applied between the two electrodes. In addition, because of the long distance between the two electrodes, some ions and electrons will recombine before they reach the electrodes. As a result, the sensitivity of the detector is reduced. Once leaving the UV window of the lamp, the high energy photons travel a very short distance before they are absorbed by the organic gas molecules. Therefore, the region that photo-ionization process actually take place is just a few millimeters in front of the UV window. Beyond that region, the photo-ionization activity decreases rapidly. Therefore, in a classical cylindrical chamber design, the effective region for photo-ionization is limited only to the space right in front of the UV window. The gas molecules far from the UV window are unlikely to be ionized.
The prior art UV lamps, although useful in connection with the novel chamber of this invention, also have certain drawbacks, such as short life and low efficiency. In my co-pending application, Ser. No. 08/061,419, filed simultaneously herewith, there is described a novel gas discharge UV lamp particularly suitable for use in the photo-ionization detector of this invention.
The basic prior art photo-ionization detector has an inherent limitation: it cannot distinguish between different gases with similar ionization energy. When the high-energy photon hits a gas molecule, if the photon energy is higher than the ionization energy of the molecule, it will ionize it. By measuring the amount of ions and comparing it with a pre-defined reference value for that gas, the gas concentration can be calculated. However, there is usually more than one type of gas which has lower ionization energy than a specific photon energy. Therefore, for a given photon energy, these gas molecules can all be ionized. It is not possible to identify a specific gas and its correct concentration if the type of gas is unknown. In addition, if there is more than one type of gas present at the same time and the photon energy is higher than the ionization energy of these gases, it is also not possible to calculate the correct concentration of each individual gas.
This lack of specificity is a major drawback of present day PID instruments. They can be used to detect a single gas type. However, the user needs to know ahead of time what type of gas is being measured, and set the instrument calibration accordingly. Traditionally, a separation column is placed in front of the PID in order to separate the gas molecules before they enter the PID for detection and measurements.
The two most critical elements in the PID instrument design are the high-energy UV light source and the ionization chamber. A good UV light source produces stable UV light output and is very reliable and rugged. It also needs to be energy efficient, if it is to be used in portable battery-powered instruments. The chamber should be designed in such a way that even a very small number of gas molecules (less than one part per million) can be measured accurately. The instrument should be capable of distinguishing the type of gas.